[llvm] [RISCV] improve `musttail` support (PR #170547)

[Matt Arsenault via llvm-commits]

================
@@ -9,12 +9,13 @@
 ; RUN: not --crash llc -mtriple riscv64-unknown-elf -o - %s \
 ; RUN: 2>&1 | FileCheck %s
 
-%struct.A = type { i32 }
+declare void @callee_musttail()
----------------
arsenm wrote:

Add new signature test, don't remove old signature test? 

https://github.com/llvm/llvm-project/pull/170547